Field of the Invention
This invention relates to downdraft stoves, and to a novel tubular grating for such stoves.
Downdrafters have been used in woodburning stoves and other combustion units for some time. A downdrafter provides uniform combustion during and after refuelling intervals, which is impossible to achieve in an updrafter with vertical filling and combustion.
The primary disadvantage of an updrafter is that combustible material is roasted prior to being ignited. First the volatives ignite, and if there are many, insufficient oxygen will be present for complete combustion. Then while the fire builds up, intensive heat chars the fuel adjacent to the embers and hot combustion products dehydrate the overyling fuel, resulting in a mixture of pyrolytic and primary combustion products, which rarely are completely burned, and continuously vary in composition. Finally, as combustion progresses to a steady state heat output, mostly charcoal is burnt, but again mosly incompletely as the amount of air admitted must be restricted to limit the rate of combustion.
In a downdrafter all the subsequent stages of combustion are simultaneous and the release of volatiles and primary combustion products are much more uniform across the burn up cycle. Fuel is ignited by radiation rather than convection of ascending fuel gas. Moisture emanates at an even rate and not all at once, thus preventing the chill and subsequent change in burnup rate and combustion gas composition associated with updrafters, when freshly stoked.
The disadvantage of the downdrafter is the inverted draft direction. The surrounding atmoshpere pushes warmer and therefore less dense gases upwards rather than downwards unless there is no upward escape and a pressure differential can be established between the air/flue gas interface and the site of the combustion in progress. In practical terms this means that a minimum amount of draft tightness has to be provided to make a downdrafter functional.
However, no downdrafter guarantees complete combustion per se. It merely provides better control of combustion by the nature of its geometry. Additional features that tend to ensure completeness of combustion are the subject of this invention.
Controlled combustion can be achieved by limiting the amount of primary air allowed into the primary combustion zone while at the same time offering ample surface contact above ignition temperature to ensure complete consumption of the oxygen.
Complete combustion can be achieved by offering surplus oxygen at a point where it will not further entertain primary combustion but still contain sufficient heat to react with primary combustion products (incompletely burned components, volatiles and pyrolites and mainly carbon monoxide).
Controlled combustion is a precondition for complete combustion. Complete combustion can only be achieved if sufficiently high temperatures and sufficient surplus oxygen are provided before the components are allowed to escape.
These two conditions of complete combustion are somewhat difficult to reconcile and are in fact the main stumbling blocks of even modern types of advanced combustion technology. The catalytic converters commonly offered today as the ne plus ultra in woodburning stoves can be considered as tokens of admitted defeat in this endeavour.
It is desired to provide a stove capable of burning combustibles of solid consistency of very varied origin, simultaneously if necessary, with substantially complete combustion for optimal net heat output and environmental suitability. The unit should be useable for combined combustion of solid and liquid and even gaseous fuels, when used in conjunction with accessories. The unit should be useable where the primary concern is heat generation, or where it is waste elimination, or where both are desired and the combustible matter is capable of generating adequate heat. The stove ideally should be capable of continuous operation with intermittent recharging and ash removal, without interruption of combustion, although solid incombustibles larger than the grating may have to be removed after burn out and cooling.